Process Pipe Fittings Knowledge

Pipe fittings are classified by purpose

Elbows 

Elbows are used to change the direction of the pipeline.

Commonly used elbows have bending angles of 90°, 45° and 180°. 180° elbows are also called U-shaped elbows. There are also elbows with special angles, but they are very few.

❶ Malleable cast iron elbows 

Malleable cast iron elbows, also known as forgeable cast iron elbows, are the most common threaded elbows. The specifications of malleable cast iron elbows are relatively small, and the commonly used specifications range from 1/2 to 4 inches. According to their different surface treatments, they are divided into galvanized and non-galvanized. This kind of malleable cast iron pipe fittings is mainly used for heating, water pipes and gas pipes. In the process, except for low-pressure pipes that need to be disassembled frequently, other material pipes are rarely used.

❷ Pressed elbows 

Pressed elbows, also known as stamped elbows or seamless elbows, are made of high-quality carbon steel, stainless acid-resistant steel and low-alloy steel seamless pipes, etc., and are pressed and formed in a special mold. Its bending radius is one and a half times the nominal diameter (R=1.5DN). In special occasions, elbows with a bending radius equal to the nominal diameter (R=1DN) can also be used. Its specification range is within DN20~600mm. Its wall thickness range is consistent with the table number of seamless steel pipes. Pressed elbows are generally stamped and processed by professional manufacturers or processing plants using standard seamless steel pipes. The bevels at both ends of the elbows should be processed before leaving the factory.

❸ Stamped and welded elbows 

Stamped and welded elbows are made by stamping a half-ring elbow with a plate through a die, and then the two half-ring elbows are assembled and welded. Its bending radius is the same as that of seamless pipe elbows, and the specification range is more than 200mm in nominal diameter.

❹ Welded elbows 

Welded elbows are also called shrimp waists or shrimp body elbows. There are two production methods: one is to cut steel plates in the processing plant, roll and weld them after cutting, and most of them are used for matching steel plate coils. The other is to cut pipes and form them by welding. The specifications are generally above 200mm. The use temperature cannot be greater than 200℃, and it can generally be made on the construction site.

❺ High-pressure elbow 

High-pressure elbow is forged with high-quality carbon steel or low-alloy steel. According to the pipeline connection form, the two ends of the elbow are processed into threads or bevels. The processing precision is very high, and the pipe thread and flange thread are required to be tightly matched and freely screwed in without loosening. It is suitable for petrochemical pipelines with pressures of 22.0 and 32.0MPa, and the commonly used specifications range from DN6 to 200mm.

Tee

Tee is a pipe fitting that connects the main pipeline and the branch pipeline. It is divided into many types according to the manufacturing material and purpose. From the specification: it can be divided into same diameter tees and different diameter tees. Same diameter tees are also called equal diameter tees; same diameter tees refer to the branch pipes with the same diameter as the main pipe. Different diameter tees refer to the branch pipes with a smaller diameter than the main pipe, so they are also called unequal diameter tees. Different diameter tees are more commonly used.

❶ Malleable cast iron tees 

The manufacturing material and specification range of malleable cast iron tees are the same as malleable cast iron elbows. They are not used much in petrochemical process pipelines. They are mainly used for indoor heating, water supply and drainage, and gas pipelines.

❷ The production of steel tees is made of high-quality pipes. After cutting, digging, heating, and drawing with a mold, and then machining, it becomes a finalized finished tee. Medium and low pressure steel finished tees are all welded when installed on site. There are two situations for the tees used in steel plate coiled pipes. One is to cut steel plates in the processing plant and roll and weld them; the other is to dig holes and take over at the installation site.

❸High-pressure tee 

There are two commonly used high-pressure tees, one is welded high-pressure tee and the other is integral forged high-pressure tee. Welded high-pressure tees use high-quality high-pressure steel pipes as materials. The manufacturing method is similar to the eyelet pipe. The hole opened on the main pipe must be consistent with the diameter of the connected branch pipe. Strict welding quality requirements usually require preheating before welding and heat treatment after welding. Its specification range is DN16~200mm, and the pressure is 22MPa and 32MPa. Integrally forged high-pressure tees are generally connected by threaded flanges. Its specification range is table number SCH160, DN15~600mm, and the table number is added with "XXS". Its pipe diameter range is DN15~300mm.

3. Reducer

The function of the reducer is to change the diameter of the pipe. From the perspective of the direction of fluid movement, most of them are from large to small, and some are from small to large, such as the reducer of the steam return pipe and the sewer pipe. It changes from small to large. Reducers are commonly known as reducers.

❶Mallony cast iron reducer

Mallony cast iron reducer is generally divided into two types, one is the internal thread reducer, also known as the external joint; the other is the pipe fittings with internal and external threads, called the bushing. Although it is not called a reducer, it plays the role of a reducer.

❷Steel reducer

Steel reducer is divided into seamless and seamed types. Seamless reducer is pressed with seamless steel pipe, and seamed reducer is made of steel plate, rolled and welded, also known as welded reducer. Both types of reducers have concentric and eccentric specifications. The bottom of the eccentric reducer has a straight edge, which can make the bottom of the pipe a horizontal plane when used, which is convenient for discharging materials in the pipe during shutdown and maintenance. The specification range of seamless reducer is DN25~600mm, and the specification range of seam welded reducer is DN200~1500mm.

Other pipe fittings

❶Head 

The head is a plug used to seal the end of the pipe. Commonly used heads are oval and flat. Oval heads are also called pipe caps. Their specifications range from DN25 to 600mm and are mostly used on medium and low pressure pipelines. Flat heads are divided into two types according to their installation position. One is that the flat head is slightly larger than the outer diameter of the pipe and is welded outside the pipe. The other is that the flat head is slightly smaller than the inner diameter of the pipe, and the head plate is placed inside the pipe for welding. The commonly used specifications range from DN15 to 200mm, and this type of head is mostly used on pipelines with lower pressures.

❷ Boss 

Boss, also known as nozzle, is a primary component of the automatic control instrumentation major on the process pipeline. It is installed by the process pipeline major, so the boss is also listed as a pipe fitting. The single-sided pipe joint used for process pipelines also belongs to this type. One end is welded on the main pipe, and the other end is either installed with other components or connected to another pipe. Its specifications range from DN15 to 200mm, and it is used for both high, medium and low pressure pipelines.

❸ Blind plate 

Blind plate, its function is to cut off the medium in the pipeline. According to the use pressure and the form of the flange sealing surface, it is divided into the following types: Smooth surface blind plate, used in conjunction with smooth sealing surface flange, its applicable pressure range is 1.0～2.5MPa. Convex surface blind plate, which has a convex surface on one side and a concave surface on the other side, is used in conjunction with concave and convex sealing surface flange. The use pressure is 4.0MPa, and the specification range is DN25～400mm. Trapezoidal groove surface blind plate, used in conjunction with trapezoidal groove sealing surface flange, the use pressure range is 6.4～16.0MPa. The specification range is DN25～300mm. "8"-shaped blind plate, also divided into smooth surface, concave and convex surface and trapezoidal groove surface three types, the use pressure is the same as the above three types of blind plates, the difference of "8"-shaped blind plate is that it combines two uses in one component, that is, the blind plate and the gasket are connected and fixed together. When the blind plate is inserted into the flange, the gasket exposed outside serves as an intuitive sign whether the pipeline is cut off. There are many materials for manufacturing "8" blind plates, which are selected according to the temperature and pressure of the medium being transported. For general low-pressure pipelines, when the temperature does not exceed 450℃, the materials used are Q235A, No. 20 steel and No. 25 steel; when the temperature is between 450 and 550℃, the materials used are 15CrMo and 1Cr5Mo. When the pressure is between 4.0 and 16.0MPa and the temperature is greater than 550℃, stainless steel should be used.

Flange, gasket and bolts 

Flange is a component that plays a connecting role on the process pipeline. This connection form has a wide range of applications, such as the connection between pipelines and process equipment, and the connection of flange valves and accessories on pipelines. The flange connection has both flexibility in installation and disassembly and reliable sealing. The medium transported by the process pipeline is of various types, and the temperature and pressure are also different, so different requirements are put forward for the strength and sealing of the flange. In order to meet the needs of process pipeline installation projects, many flanges with different structures and pressures have emerged.

The following is a brief introduction to various flanges:

❶ Flat-welded flange 

Flat-welded steel flange is the most commonly used type for medium and low pressure process pipelines. This type of flange and pipe fixing form is to put the flange on the pipe end, weld the inner and outer ends of the flange, and fix the flange. The applicable nominal pressure does not exceed 2.5MPa. Flat-welded flanges used for carbon steel pipeline connections are generally made of Q235A and No. 20 steel plates; flat-welded flanges used on stainless acid-resistant steel pipelines are made of stainless acid-resistant steel plates of the same material as the pipe. The sealing surface of the flat welded steel flange is generally smooth, and there are shallow grooves on the sealing surface. It is usually called the waterline, as shown in the figure:

Flat welded steel flange figure

The specification range of the flat welded steel flange is as follows: DN10~2000mm for nominal pressure PN0.25, 0.6MPa; DN10~600mm for PN1.0~2.5MPa.

❷ Butt-weld flange 

Butt-weld steel flange, also known as high neck flange or large tail flange. It has high strength and is not easy to deform, has good sealing performance, has a variety of sealing surfaces, and is suitable for a wide range of pressures. ① Smooth face welding flange, its nominal pressure is PN2.5MPa or less, the specification range is DN10~2000mm, the nominal pressure PN2.5MPa, the specification range is DN10~1000mm, the nominal pressure PN4.0MPa, the specification range is DN10~600mm, as shown in the figure:

Smooth face welding flange Figure 

② Concave and convex sealing face welding flange, due to the tightness of the concave and convex sealing surface, the pressure it bears is large. The sealing surface of each flange must be one concave surface and the other convex surface. The commonly used nominal pressure range is PN1.6～4.0MPa, and the specification range is DN10～600mm, as shown in the figure:

Concave and convex sealing face welding steel flange figure

③ Tongue and groove sealing face welding flange, this flange has good sealing performance, and the structure is similar to the concave and convex sealing face flange, and a pair of flanges must be used in pairs. The nominal pressure range is PN1.6~4.0MPa, the specification range is DN10~600mm; PN6.4~10MPa, the specification range is DN10~400mm, as shown in the figure:

Tongue and groove face welding flange diagram

④ Trapezoidal groove sealing face welding flange, this flange is commonly used in oil industry pipelines, with high pressure, the commonly used nominal pressure is PN6.4, 10.0MPa, the specification range is DN10~400mm; PN16MPa, the specification range is DN10~300mm. As shown in the figure:

Trapezoidal groove face welding steel flange diagram

The above-mentioned various butt welding flanges are only distinguished by the different forms of their sealing surfaces. From the perspective of installation, no matter what form of butt welding flange, the connection method is the same, so the labor, materials and machine shifts consumed are basically the same. However, due to the different forms of sealing surfaces, the processing and manufacturing costs of flanges vary greatly. Therefore, when preparing the general (preliminary) budget, special attention should be paid to the price of the flange itself.

❸ Loose-sleeve steel pipe flange 

This flange is not directly welded to the pipe, but uses the pipe flange or welding ring as the sealing contact surface. The loose-sleeve flange plays a fastening role and is mostly used on non-ferrous metals such as copper, aluminum and lead and stainless acid-resistant steel pipes. Its biggest advantage is that the flange can move freely, which makes it very convenient to bolt the flange. The welded ring loose-sleeve steel pipe flange is suitable for pressure PN0.6~2.50MPa, and its specification range is DN10~600mm; the pressure PN4.0MPa, the specification range is DN10~300mm, and the flat welded ring loose-sleeve steel pipe flange is only suitable for lower pressure, PN0.6~1.6MPa, and its specification range is DN10~600mm. The pipe flange loop method is only suitable for PN06MPa and below. The welded ring loose-sleeve flange is shown in the figure:

Welded ring loose-sleeve steel pipe flange figure

❹Threaded flange 

Threaded flange is a flange connected to the pipe end thread with a threaded flange, which is available in high pressure and low pressure. Low-pressure threaded flanges include steel and cast iron. This type of flange was widely used in the early days of the founding of the People's Republic of China when welding technology was very poor. With the development of industry, low-pressure threaded flanges have been replaced by flat welding flanges and are basically not used except in special cases. High-pressure threaded flanges are widely used in the connection of modern industrial pipelines. The seal is formed by the pipe end and the lens gasket, and the precision processing requirements of the contact surface between the thread and the pipe end gasket are very high. The characteristics of this flange are that the flange does not contact the medium in the pipe and the installation is relatively convenient. The applicable pressure is PN22.0 and PN32.0MPa, and its specification range is DN6~250mm.

High-pressure pipeline flange connection structure type diagram

❺ Flange cover 

The flange cover is a component used in conjunction with the flange. It plays a sealing role at the pipe end like the head. The sealing surface is smooth and convex and concave, and its specifications and applicable pressure range are consistent with the matching flange.

2. Flange gasket

Leakage is the main form of pipe flange failure, which is related to many factors such as the sealing structure type, the stiffness of the connected parts, the performance of the seal, operation and installation.

The gasket is the main seal of the flange connection, so the correct selection of gaskets is also the key to ensure that the flange connection does not leak. There are many types of flange gaskets according to the corrosiveness, temperature, pressure and flange sealing surface of the medium transported by the pipeline. Pipe flange gaskets include non-metallic gaskets, semi-metallic gaskets and metal gaskets.

❶Rubber asbestos pads 

Rubber asbestos pads are the most used gaskets for flange connections and can be used for many media, such as steam, gas, air, salt water, acid and alkali. The thickness of rubber asbestos pads is not unified in various professions, and 3mm is usually used. For flanges with a nominal diameter of less than 100mm, the gasket thickness shall not exceed 2.5mm. The use pressure of the gasket: when used for smooth sealing surface flange connections, it shall not exceed 2.5MPa. There are two types of rubber asbestos pads commonly used in the oil refining industry: one is an oil-resistant rubber asbestos pad, which is suitable for temperatures below 200°C and nominal pressures below 2.5MPa, and for conveying general oil products, liquefied hydrocarbons, propane and acetone and other media. High-temperature oil-resistant rubber asbestos pads can be used at temperatures of up to 350-380°C. The other is a medium-pressure rubber asbestos pad, which can be used for steam, condensate, water, air and other media at 200°C and PN below 2.5MPa. Since asbestos has been proven to be carcinogenic, gaskets containing asbestos are now basically no longer used.

❷ Rubber pads 

Rubber pads are gaskets made of rubber sheets. They have a certain degree of corrosion resistance and are often used for pipe flange connections with temperatures below 60°C and pressures not exceeding 1.0MPa, conveying low-pressure water, acids and alkalis and other media. The characteristic of this gasket is that it uses the elasticity of rubber to achieve a better sealing effect, so it is also commonly used in the installation of cast iron flange valves.

❸ Wound gasket

Wound gaskets are referred to as wound gaskets, which are wound with metal steel strips and non-metallic filler strips. Advantages: This type of gasket has the advantages of simple manufacturing, low price, full utilization of materials, and good sealing performance. It is widely used in petrochemical process pipelines. Applicable parameters: The applicable nominal pressure is below 4.0MPa, and the applicable temperature range is 450℃ for 08 steel and 540℃ for wound gaskets made of 0Cr13 steel strips. The thickness of the gasket is generally 4.5mm. When the diameter is greater than 1000mm, the thickness of the gasket is 6-7mm. The thickness of the positioning ring is about 3mm. This type of gasket is mostly used for smooth surface flange connections, and its sealing surface does not need to be waterlined. Some wound gaskets also have positioning rings to prevent the gasket from deviating from the center of the flange. The materials of metal steel strips include 08 steel, 0Cr13 steel, and 1Cr18Ni9Ti steel; the materials of non-metallic strips include characteristic asbestos, flexible graphite, polytetrafluoroethylene, etc.

❹ Toothed gaskets

Toothed gaskets are made of various metals, including ordinary carbon steel, low alloy steel and stainless acid-resistant steel, with a thickness of 3 to 5 mm. It uses concentric toothed dense grooves to contact the flange sealing surface and form a multi-pass seal, so it has good sealing performance and is often used for the connection of concave and convex sealing surface flanges. Applicable parameters: The maximum nominal pressure can reach 16.0MPa, which is suitable for parts with high working temperatures. For example, the toothed gasket made of 0Cr13 material has an applicable temperature of up to 530℃.

❺ Metal gaskets

There are many types of metal gaskets. According to the shape, there are metal flat gaskets, with elliptical and octagonal metal gaskets and lens-type gaskets in cross-sectional area. According to the manufacturing material, there are low carbon steel, stainless acid-resistant steel, copper, aluminum and lead. Metal flat gaskets are mostly used for smooth surface flat welding flanges, and they withstand lower temperatures and pressures. Elliptical and octagonal metal gaskets are mostly used for trapezoidal groove butt welding flanges, with a nominal pressure range of 6.4 to 22.0MPa. Although this type of gasket has good sealing performance, it is complex to manufacture and requires high precision. There is a principle for the use of metal gaskets, that is, the hardness of the gasket surface must be lower than the hardness of the flange sealing surface. The selection of gaskets should be determined according to the temperature, pressure, corrosiveness of the medium transported by the pipeline and the sealing form of the connecting flange. More professional gaskets, such as lens gaskets, are suitable for high-pressure flange connections.

3. Bolts for flanges

There are two types of bolts used to connect flanges: single-head bolts and stud bolts. Their threads are generally triangular metric coarse threads.

❶ Single-head bolts 

Single-head bolts are also called hexagonal head bolts. Single-head bolts are divided into semi-refined and refined. Semi-refined single-head bolts are the most commonly used in medium and low pressure process pipelines. Commonly used materials for manufacturing single-head bolts are Q235A, 35 steel and 25Cr2MoVA. Commonly used for flange connections with a nominal pressure of less than 2.5MPa. The applicable temperature depends on the material of the bolt. For example, the applicable temperature of bolts made of 35# steel can reach 350°C; the applicable temperature of bolts made of 25Cr2MoVA steel can reach 570°C.

❷ Stud bolts 

Most of the stud bolts used on process pipelines are equal-length double-headed refined bolts. Suitable for flange connections with high temperature and pressure. The materials include 35# steel, 30CrMoA, 35CrMoA, 25Cr2MoVA, 0Cr19Ni9, 0Cr15Ni25Ti2MoAlVB and 37SiMn2MoVA, etc. The nominal pressure range is 1.6~32.0MPa, and the applicable temperature can reach 700°C.

❸ Nuts 

Nuts are collectively referred to as hexagonal nuts. There are two types: semi-refined and refined. According to the nut structure, they can also be divided into type A and type B. Semi-refined single-head bolts mostly use type A nuts; refined stud bolts mostly use type B nuts. Nuts and bolts should be used together, but the hardness of the nut manufacturing material cannot exceed the hardness of the bolt material.